Why it feels like the only option
The equity is in the house, and the deposit for the next one has to come from somewhere. If the only route you know is a sale, then selling first is the plan, and the consequence is either moving twice or writing an offer that depends on your sale closing.
In a market with any competition, an offer contingent on selling is the weakest offer on the table. That is the real cost of this belief, not inconvenience, but losing houses.